With any internal combustion engine it is desirable to treat exhaust gases so that they can be safely discharged into the atmosphere. In some engines, particularly of the diesel type, among the most prevalent operating problems is the presence of particulates which are carried in the exhaust gas stream.
Primarily, the particulates are normally bits of carbon. They result from incomplete combustion of the hydrocarbon fuel under certain engine operating conditions. However, the operating efficiency of the engine is also a contributing factor to the amount of carbon produced.
The presence of relatively large amounts of carbon particles in any exhaust gas stream is evidenced by a dark, smoky, undesirable effluent. Such smoke is not only offensive aesthetically; in large quantities it can be unhealthy.
Means have been provided and are known to the prior art, for the elimination or minimization of the particulate content in exhaust discharge streams. However, it has been found that while the particulates can be eliminated by a suitable filter of proper construction, eventually the latter can become saturated and/or inoperable due to excessive particulate accumulations.
It is further known that the overall engine exhaust gas treating process can be expedited. This is achieved not only by passing the hot gas stream through a filter medium, but by providing the filter with a catalyst which will promote combustion of retained particles.
It should be appreciated that the generation of carbon particles is prevalent under all diesel engine operating conditions. It is further appreciated that the quantity and quality of an exhaust gas stream created in any internal combustion engine will vary in accordance with the operating characteristics of the engine.
For example, the temperature range experienced by a diesel exhaust gas stream can vary between slightly above ambient air temperature, and temperatures in excess of 1200.degree. F. When the exhaust gas is hot enough, carbon particles trapped in a filter will be combusted. However, engine operating conditions at which this rejuvenation can occur is not always attainable in diesel passenger cars, buses or the like.
Where it is found that an engine continuously operates under such circumstances that particulates are continuously produced and accumulated in the filter, the particulate trapping filter bed must be rejuvenated with a degree of consistency.
When the exhaust is sufficiently hot, rejuvenation will consist of merely introducing the hot exhaust gas stream, containing sufficient oxygen, into the filter bed to contact and incinerate retained carbon particles. The combustion of any large, contained carbon accumulation can however, produce temperatures in excess of that of the exhaust gas. The result is that at such excessive temperatures, the filter bed is susceptible to thermal shock, damage or distortion.
Toward achieving an improved and controlled rate of carbon removal from an exhaust gas stream without incurring damage to the filter, the unit presently disclosed is provided.
The instant system thus constitutes in brief, a reaction chamber or filter bed which comprises in part a catalytic segment or section through which the exhaust gas stream flows. This catalytic surface can be incorporated within the particle trapping bed, or can be disposed at the upstream end thereof.
To assure that the main filter bed remains functional in spite of engine operating conditions, a portion of the exhaust gas stream is periodically preheated within an electrically powered heating zone.
This stream is passed into contact with the catalytic segment, thereby raising the temperature of a part of the catalyst segment to the catalyst "lightoff" temperature.
Supplementary fuel is preheated by being brought into contact with a hot surface or surfaces of the filter. The heated fuel is then injected into the heated portion of the exhaust gas to form a fuel/exhaust gas mixture. When the latter mixture contacts the heated catalyst, it will ignite. When the oxidizing action within the catalyst section becomes self-sustaining, the initial electrical heating of the exhaust gas stream can be discontinued.
In summary, the main filter bed will be regularly and at periodic intervals, purged or rejuvenated by hot exhaust gas from the catalyst section. Such treatment, if repeated at predetermined times will preclude carbon accumulations which, if not disposed of, might otherwise lead to thermal stress or damage to the filter bed at such time as the accumulation is combusted.
It is therefore an object of the invention to provide a filter of the type disclosed which is capable of retaining combustible particulates from an exhaust gas stream, and of being periodically rejuvenated by incinerating the particulates.
A further object is to provide a particulate filter of the type disclosed which is capable of removing solid combustible elements from an exhaust gas stream while permitting periodic rejuvenation of the filter element.
A still further object is to provide a filter unit for an internal combustion engine, which filter is periodically rejuvenated by supplemental heating means and by introduction of a preheated flow of fuel to the filter bed while the engine is operating at conditions that do not ordinarily result in exhaust gas temperatures sufficiently high to initiate combustion of the supplementary fuel.